An electronic circuit having a processor will require clock circuitry such as a phase-locked loop (PLL) which generates a clock signal for the circuit. The clock signal may subsequently be divided and/or branched multiple times throughout the circuit, but the PLL is usually the master source of the clock signal.
A PLL generates the clock signal by reference to a reference oscillator, typically a crystal oscillator mounted on the same board as the processor chip. For a processor, the reference oscillator frequency is typically of a lower order of magnitude than the clock frequency to be generated.
The PLL works by branching off its output clock signal and dividing the frequency by the desired factor, then feeding back the divided clock signal for comparison with the phase of the reference signal. An error signal is generated based on the comparison, and used to adjust the output clock frequency. Thus the output clock signal is held at a frequency which is higher than the reference frequency by the division factor of the loop.
Because processor design is a long and expensive process, it is desirable to design a processor with different applications and/or customers in mind, and/or to anticipate possible future modifications. One aspect of this design philosophy is to design a chip that can be relatively easily modified at the manufacturing stage to accommodate different frequency reference oscillators. This is currently done by providing the chip with one or more fuse latches such as electronically programmable “e-fuses”. Typically each fuse represents one bit, e.g. with a blown fuse representing a logic-one and an un-blown fuse representing a logic-zero, such that a binary value can be written into one or more of such latches. The PLL is arranged so that the division factor in its feedback loop is dependent on the value written to the fuse latches at chip manufacture, thus allowing different frequency reference oscillators to be chosen for a given output frequency.
However, this technique is still not as flexible as it could be, because the choice must be permanently fixed upon manufacture. It would be advantageous to provide a more flexible mechanism.